Among bipolar transistors (BTs), application of AlGaAs/GaAs heterojunction bipolar transistor (HBT) to digital devices or circuits for micrometer wave and/or millimeter wave communication is expected because of its ultra high-speed operation.
Further increase in operation speed of HBT requires shortening of electron transit time. Attention is to be paid that electron transport time through a collector depletion region occupies a great proportion of electron transit time. Thus, shortening of the electron transport time is particularly required. FIG. 8 shows a representative band diagram of a conventional npn HBT.
As readily seen from the band diagram, electrons gain energy from base-collector depletion region where, under biased condition, high electric field builds up. The field-assisted electrons transfer to higher energy valley with larger effective mass, for example, L valley, resulting in an increase in effective mass. Reduction in mobility results. Thus, it is required to suppress the intensity of electric field to a level weak enough to prevent the transfer of electrons. According to one conventional proposal, fabricating a collector layer of HBT with semiconductor having the p-conductive type lowers the intensity of electric field across base-collector interface as shown in FIG. 9. As shown in FIG. 10, according to another conventional proposal, the intensity of electric field is lowered by fabricating a collector layer with semiconductors having i-conductive type and p-conductive type to alter band diagram of the collector layer.
According to the conventional proposals, impurity concentration in the collector layer changes from the usual n-conductive type to i-conductive type and p-conductive type so as to shift peak of intensity of electric field from the base-collector interface to the sub-collector side. However, in the case of npn HBT, the modification of impurity distribution in collector layer causes undesired device characteristics because the collector layer is usually of the n-conductive type. For high speed operation, it is desired to maintain injection of high concentration of current to shorten time required for charging emitter. However, in the case of collector that is not doped sufficiently with n-conductive type material, injection of high concentration of current into the collector brings about Kirk effect, thereby failing to accomplish desired level of high speed operation of HBT. In summary, even if a complicated, in structure, collector layer is employed to accomplish a desired contour of band diagram, an actual contour of band diagram is subject to an unexpected deviation from the desired contour during operation of HBT due mainly to occurrence of Kirk effect.